Cathode-ray tube and process for producing the same

ABSTRACT

In order to provide a method of producing a cathode ray tube in a manner which well matches with a usual technique of producing a cathode ray tube by forming a spiral high-resistor on an inner face of a glass tube, in an electron gun of which a main focusing lens is configured by the spiral high-resistor, the step of fritting seal rings to both ends of the glass tube in which a hole is opened in the center portion; the step of applying a high-resistor paste to the glass tube, drying the paste, and then forming a spiral structure in the high-resistor film; the step of firing at 420 to 550 deg. C.; and the step of combining other electron gun parts to form the electron gun are included, and the spiral high-resistor is formed by a high-resistor paste in which ruthenium oxide is added to a glass material having a softening point that is lower than the annealing point of the glass tube.

TECHNICAL FIELD

The invention relates to a cathode ray tube for projection type anddirect viewing type in which a spiral high-resistor is used as a mainfocusing lens, and also to a method of producing it.

BACKGROUND OF THE INVENTION

It is known that an electron gun in which a main focusing lens isconfigured by a spiral high-resistor can attain high resolution in acathode ray tube produced by using the same.

As a method of producing such a main focusing lens of an electron gunfor a cathode ray tube, as shown in Japanese patent publication (Kokai)No. HEI6-275211, there is a method using a paste containing rutheniumoxide (RuO₂) and glass. In this method, a resistance material consistingof the ruthenium oxide and a glass paste is formed into a spiral shapeon the inner face of a cylindrical tube of insulative ceramics or thelike. In another method, after a resistance material consisting of theruthenium oxide and the glass paste was uniformly applied to the innerface of a cylindrical tube, it is formed into a spiral resistor memberby trimming method or other method. Thereafter, firing is conducted at850 deg. C. for 10 minutes, with the result that a spiral high-resistorhaving a resistance of 100 MΩ to 10 TΩ is obtained. In order toelectrically connect the thus formed main focusing lens to otherelectrode, a cylindrical holder made of a metal such as stainless steelis fitted into the inner face of an end portion of the cylindrical tube.In the cylindrical holder, pairs of projections which are opposed toeach other are disposed in three portions. Among the projections, innerones are contacted with the spiral high-resistor formed on the innerface of the cylindrical tube.

Another method of producing a main focusing lens is disclosed inJapanese patent (Kokoku) No. HEI4-23402. In the method, a resistor layeris formed on the inner face of a hollow tube which is made of glass andto which a metal part is welded, by applying a suspension containingruthenium hydroxide (Ru(OH)₃) and glass particles and then drying it.Thereafter, the resistor layer on the inner face of the hollow tube ismachined into a spiral shape, and then heated to 400 to 600 deg. C., sothat ruthenium hydroxide is changed to ruthenium oxide, and a spiralresistor of a composition in which ruthenium oxide fuses with glassparticles in the resistor layer is obtained.

However, the above-mentioned methods of the prior arts have severalproblems in formation of a spiral high-resistor having uniformproperties.

First, the resistance material used in the method disclosed in theJapanese patent publication (Kokai) No. HEI6-275211 is a thick-filmresistor material for a chip resistor which is conventionally used in anelectronic circuit board or the like. Therefore, the firing temperatureis considerably high or 850 deg. C., and a glass tube or a ceramic tubewhich can withstand the temperature must be used. Although a kind ofsuch a glass tube is a quartz glass tube, a quartz glass tube has a verysmall coefficient of thermal expansion as described later, therebyproducing a problem in connection with other member having a largecoefficient of thermal expansion. In the case where a ceramic tube isused, since a molding accuracy is inferior to that of a glass tube, aworking process must be done during or after a process of molding theceramic tube in order to improve the accuracy, resulting in a high cost.

Specifically, in the method disclosed in the Japanese patent (Kokoku)No. HEI6-275211, ruthenium oxide is used as the high resistancematerial. Since the firing temperature is 850 deg. C., quartz glass isused; but the quartz glass is expensive. The coefficient of thermalexpansion is 5.5×10⁻⁷ /deg. C. and very smaller in comparison with thecoefficient of thermal expansion of usual glass for the cathode raytube. By contrast, in the case where the ceramic tube is used, since thesurface roughness is so large as 1 to 2 μm, it is difficult to form aspiral with high accuracy on the inner face of the ceramic tube. Whenthe surface of the ceramic tube is smoothed by cutting the innersurface, it becomes very expensive. Furthermore, since a metal partcannot be welded to the ceramic tube with frit glass, in order to bondthe metal part such as a metal electrode to the ceramic tube, a specialworking process is required.

In the method disclosed in HEI4-23402 in which ruthenium hydroxide isused, the suspension has a low viscosity and hence it is difficult toform a layer of such a thickness as larger than about 1 to 1.5 μm anduniform. Furthermore, glass flows during the process of thermallydecomposing ruthenium hydroxide, which is an insulator, by heating (400to 600 deg. C.) to deposit ruthenium oxide which is a conductor. At thistime, very fine ruthenium oxide particles of 0.01 to 0.03 μm aredeposited around glass particles so as to form a resistor. In such acase, when a high resistance of, for example, 20 GΩ is to be obtained,the firing temperature dependency becomes large. In other words, therewas a problem in that even a small variation of the firing temperaturecaused the resistance to be largely changed.

Furthermore, when the metal part is welded to the hollow glass tube at ahigh temperature of 800 deg. C. or higher, an oxide film is formed onthe surface of the metal part and charges are generated in the surface.Therefore, the electric field of the main focusing lens becomesunstable. The inventor has found a problem of generation of suchphenomenon that the charges cause spot shape to swingingly deform andimpair the resolution of the display screen in operation of the cathoderay tube.

Moreover, there is a possibility that the oxide film impairs theconnection state between the metal part and the resistor member.

In the connection part of the metal part and the hollow glass tube,moreover, molten glass may rise up to form an annular projection. Whenthe annular projection is formed, the thickness of a resistor applied tothe portion is reduced hence inducing a possibility of conductionfailure.

The invention purposes to provide a high-resolution cathode ray tubehaving a main focusing lens using a spiral which solves theabove-discussed problems and has a stable resistance, and a method ofeconomically producing it.

To comply with this, materials of parts are selected so that a firingprocess can be conducted at about 450 deg. C. of a firing temperature inproduction of the cathode ray tube of the prior art.

It is an object to prevent variation of the spot shape from occurring byavoiding formation of oxide film on the surface of a metal part or byremoving the formed oxide film, thereby improving the resolution.

BRIEF SUMMARY OF THE INVENTION

According to the invention, the method for producing a cathode ray tubehaving a spiral, which is formed on an inner face of a glass tube andfunctions as a main focusing lens for an electron gun, comprises thesteps of:

applying a high-resistor paste on the inner face of the above-mentionedglass tube, thereby forming a high-resistor film;

forming a spiral groove in the above-mentioned high-resistor film; and

firing the high-resistor film having the above-mentioned spiral grooveat a temperature of 440 to 460 deg. C. to obtain a spiral high-resistor.

As the high-resistor paste, high-resistor with a resistance not lowerthan 0.8 GΩ and not higher than 100 GΩ after firing in theabove-mentioned temperature range is used.

According to the above-mentioned structure and production method, it isnot required to use a ceramic tube which has high heat resistance as acylindrical tube for a main focusing lens.

The high-resistor paste for forming the above-mentioned spiralhigh-resistor has a feature that a high-resistor material in whichruthenium oxide is added to glass powder having a softening point thatis lower than the annealing point of the glass tube is used. Because ofthis feature, distortion of the glass tube can be eliminated.

Preferably, the glass tube, frit material, a seal ring, and thehigh-resistor material which constitute the above-mentioned electron gunfor the above-mentioned cathode ray tube have a coefficient of thermalexpansion in the range of 85 to 105×10⁻⁷ /deg. C. According to this, itis possible to prevent a crack and separation in the high-resistor filmdue to the heat treatment. It is also possible to prevent the seal ringfrom being separated from the glass tube.

The resistance of the above-mentioned spiral high-resistor is selectedin the following manner. Namely, the resistance is set so that, when adifference voltage between the anode voltage and the focus voltage ofthe cathode ray tube is applied to the spiral high-resistor, the currentflowing through the above-mentioned spiral high-resistor is in the rangeof 0.25 μA to 30 μA. As a result of this setting, a potentialdistribution which is necessary and preferable can be obtained andvariation of the focus voltage is prevented from occurring.

In another aspect of the invention, the method of producing a cathoderay tube has a main focusing lens, in which a high-resistor of ahigh-resistance material having a thermal expansion coefficient of 36 to105×10⁻⁷ /deg. C. is formed into a spiral shape on an inner face of aglass tube of borosilicate glass or soda glass material having a thermalexpansion coefficient of 36 to 105×10⁻⁷ /deg. C. and a volume resistanceof 1×10¹⁰ to 1×10¹² ohm·cm, and comprises the steps of:

disposing seal rings being metal parts which are used for electricalconnection with other electrode parts to both ends of theabove-mentioned glass tube;

applying a high-resistor paste to the above-mentioned glass tube,thereby forming a high-resistor film;

forming the above-mentioned high-resistor film into a spiral structure;

firing the above-mentioned glass tube at 420 to 550 deg. C.; and

combining other electrode parts with the above-mentioned glass tube toform an electron gun.

A preferred species of the above-mentioned production method furthercomprises a step of disposing a focusing voltage supply portion in thevicinity of the center of the glass tube.

More preferably, the above-mentioned high-resistor paste is ahigh-resistor material in which ruthenium oxide is added to a glassmaterial having a softening point that is lower than the annealing pointof the above-mentioned glass tube.

The glass material contains 25 to 40 wt. % of a filler made of amaterial of at least one of ZrO₂, SiO₂, and Al₂ O₃.

In a species of the method of producing the above-mentioned cathode raytube, it is characterized in that the resistance is set so that, when adifference voltage between the anode voltage and the focus voltage isapplied to the above-mentioned spiral high-resistor, the current flowingthrough the spiral high-resistor is in the range of 0.25 μA to 30 μA.

In a further preferred species of the method of producing theabove-mentioned cathode ray tube, the method further comprises a stepof, in order to bond a metal part for applying a predetermined potentialto the spiral high-resistor to the glass tube, welding the metal part tothe above-mentioned glass tube by using a frit, and it is characterizedin that the above-mentioned glass tube, the above-mentioned frit, theabove-mentioned metal part, and the above-mentioned material of thespiral high-resistor have a thermal expansion coefficient in the rangeof 36 to 105×10⁻⁷ /deg. C. According to these matters, it is possible toprevent a crack and separation due to the heat treatment from occurring.

Furthermore, the method of producing the above-mentioned cathode raytube is characterized in that it further comprises the step of, in orderto bond a metal part for applying a predetermined potential to theabove-mentioned spiral high-resistor to the above-mentioned glass tube,welding the glass tube to the metal part by melting a frit or the glasstube itself in a reducing gas atmosphere or an inert gas atmosphere.According to this configuration, the metal part is prevented from beingoxidized.

Furthermore, in the method of producing the above-mentioned cathode raytube, in the step of, in order to bond a metal part for applying apredetermined potential to the above-mentioned spiral high-resistor tothe above-mentioned glass tube, welding it to the above-mentioned metalpart by melting a frit or the glass tube itself, it is characterized inthat the above-mentioned metal part has a film for preventing oxidation.

It is characterized in that the film for preventing oxidation is a filmwhich is formed by one of deposition of gold, gold plating, and nickelplating.

In a further aspect of the invention, the method of producing a cathoderay tube having a spiral high-resistor which is formed on an inner faceof a glass tube and which functions as a main focusing lens for anelectron gun is characterized by comprising the steps of: in order tobond a metal part for applying a predetermined potential to theabove-mentioned spiral high-resistor to the above-mentioned glass tube,welding the glass tube to the above-mentioned metal part by melting afrit or the glass tube itself; and, after bonding the above-mentionedglass tube to the above-mentioned metal part, removing an oxide film onthe surface of the above-mentioned metal part.

Preferably, the step of removing the above-mentioned oxide film is areduction step by means of heating in a hydrogen or hydrogen-mixture gasatmosphere.

The method of producing a cathode ray tube of the invention ischaracterized in that the hydrogen or hydrogen-mixture gas atmosphere isformed by passing hydrogen or a hydrogen-mixture gas through astraightening mesh. The entrance of oxygen is prevented from occurringby burning hydrogen.

In the method of producing a cathode ray tube of the invention, the stepof removing the above-mentioned oxide film is characterized bycomprising the step of immersing in hydrochloric acid or a hydrochloricacid rust removing agent.

Furthermore, the step of removing the above-mentioned oxide film ischaracterized by comprising the step of, after immersing in hydrochloricacid or a hydrochloric acid rust removing agent, immersing in aneutralizing rust preventing agent.

The step of removing the above-mentioned oxide film is characterized bycomprising the step of mechanically shaving off the oxide film. When theoxide film is removed by either of the methods, it is possible torealize a satisfactory connection state between the metal part and thehigh-resistor film.

In a further aspect of the invention, the method of producing a cathoderay tube having a spiral high-resistor which is formed on an inner faceof a glass tube and which functions as a main focusing lens for anelectron gun is characterized by comprising the steps of: in order tobond a metal part for applying a predetermined potential to theabove-mentioned spiral high-resistor to the above-mentioned glass tube,welding the glass tube to the above-mentioned metal part by melting afrit or the glass tube itself; and flattening the bonded portion betweenthe above-mentioned metal part and the above-mentioned glass tube.

In a further aspect of the invention, the method of producing a cathoderay tube having a spiral high-resistor which is formed on an inner faceof a glass tube and which functions as a main focusing lens for anelectron gun is characterized by comprising steps of: in order to bond ametal part for applying a predetermined potential to the spiralhigh-resistor to at least one of open ends of the glass tube, weldingthe glass tube to the metal part by melting the glass tube itself; andchamfering the inner face in the vicinity of the open end of the glasstube.

In a further aspect of the invention, the method of producing a cathoderay tube having a spiral high-resistor which is formed on an inner faceof a glass tube and which functions as a main focusing lens for anelectron gun is characterized by comprising the step of, in order tobond a metal part for applying a predetermined potential to theabove-mentioned spiral high-resistor to at least one of open ends of theabove-mentioned glass tube, welding the glass tube to theabove-mentioned metal part by melting the glass tube itself, the innerdiameter of the above-mentioned glass tube being larger than the innerdiameter of the above-mentioned metal part. According to each of theabove-mentioned methods, it is possible to prevent an annular projectionfrom being formed in a connecting portion between the metal part and theglass tube and the connected portion can be flattened.

The cathode ray tube in a further aspect of the invention having a mainfocusing lens structure in which a high-resistor is formed into a spiralshape by a high-resistance material having a thermal expansioncoefficient of 36 to 105×10⁻⁷ /deg. C. is characterized by comprising:metal parts which are used for electrical connection with otherelectrode parts disposed at both ends of the above-mentioned glass tube;a high-resistor film which is obtained by forming a high-resistor pasteinto a spiral shape on the above-mentioned glass tube and firing at 420to 550 deg. C.; and other electrode parts attached to theabove-mentioned glass tube.

The above-mentioned cathode ray tube further comprises a focusingvoltage supply portion which is disposed in the vicinity of the centerof the above-mentioned glass tube.

In the above-mentioned cathode ray tube having the spiral high-resistorwhich is formed on the inner face of the glass tube and which functionsas the main focusing lens for the electron gun, it is characterized inthat it comprises a metal part which is bonded to the above-mentionedglass tube by using a frit and which applies a predetermined potentialto the above-mentioned spiral high-resistor, and the thermal expansioncoefficients of the glass tube, the frit, the metal part, and a materialof the spiral high-resistor are in the range of 36 to 105×10⁻⁷ /deg. C.

In the cathode ray tube having a spiral high-resistor which is formed onan inner face of a glass tube and which functions as a main focusinglens for an electron gun, it is characterized in that it comprises ametal part which is welded to the above-mentioned glass tube by meltinga frit or the glass tube itself and which applies a predeterminedpotential to the above-mentioned spiral high-resistor, and the bondingof the above-mentioned glass tube and the above-mentioned metal part isconducted in a reducing gas atmosphere or an inert gas atmosphere.

In the cathode ray tube having a spiral high-resistor which is formed onan inner face of a glass tube and which functions as a main focusinglens for an electron gun, it is characterized in that it comprises ametal part which is welded to the glass tube by melting a frit or theglass tube itself and which applies a predetermined potential to theabove-mentioned spiral high-resistor, and the above-mentioned metal parthas a film for preventing oxidation.

The above-mentioned film for preventing oxidation is a film formed byone of deposition of gold, gold plating, chromium plating, and nickelplating.

The cathode ray tube of the invention having a spiral high-resistorwhich is formed on an inner face of a glass tube and which functions asa main focusing lens for an electron gun is characterized in that itcomprises a metal part which is welded to the above-mentioned glass tubeby melting a frit or the glass tube itself and which applies apredetermined potential to the above-mentioned spiral high-resistor,and, after bonding the above-mentioned glass tube to the above-mentionedmetal part, an oxide film on the surface of the above-mentioned metalpart is removed.

The cathode ray tube in a further aspect of the invention having aspiral high-resistor which is formed on an inner face of a glass tubeand which functions as a main focusing lens for an electron gun ischaracterized in that it comprises a metal part which is welded to theabove-mentioned glass tube by melting a frit or the glass tube itselfand which applies a predetermined potential to the above-mentionedspiral high-resistor, and a bonding portion of the glass tube which iswelded to the metal part is flattened.

The cathode ray tube in a further aspect of the invention having aspiral high-resistor which functions as a main focusing lens for anelectron gun and which is formed on an inner face of a glass tube ischaracterized in that it comprises a metal part which is welded to atleast one of open ends of the above-mentioned glass tube by melting theglass tube itself and which applies a predetermined potential to theabove-mentioned spiral high-resistor, and the inner face in the vicinityof the open end of the glass tube is chamfered.

The cathode ray tube in a further aspect of the invention having aspiral high-resistor which functions as a main focusing lens for anelectron gun and which is formed on an inner face of a glass tube ischaracterized in that it comprises a metal part which is welded to atleast one of open ends of the above-mentioned glass tube by melting theglass tube itself and which applies a predetermined potential to theabove-mentioned spiral high-resistor, and the inner diameter of theabove-mentioned glass tube is made larger than the inner diameter of theabove-mentioned metal part. According to the configurations, it ispossible to prevent an annular projection from being formed in aconnecting portion between the metal part and the glass tube, and toflatten the connected portion.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The foregoing summary, as well as the following Detailed Description ofthe Preferred Embodiments of the Invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings andembodiment which is presently preferred. It should be understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown. In the drawings:

FIG. 1(a) is a section view of a cathode ray tube using an electron gunof Embodiment 1 of the invention.

FIG. 1(b) is an enlarged section view of the center portion of a mainfocusing lens.

FIG. 1(c) is an enlarged section view of the center portion of anotherexample of a main focusing lens.

FIG. 2 is a flowchart of steps of producing the cathode ray tube usingthe electron gun of Embodiment 1.

FIG. 3(a) is a section view showing a state in which a high-resistorapplied to a glass tube of Embodiment 1 is cut into a spiral shape.

FIG. 3(b) is an enlarged view of a spiral cutting region.

FIG. 3(c) is a plan view of a seal ring.

FIG. 4 is a side section view of an apparatus for applying ahigh-resistor paste by a dip method.

FIG. 5 is a graph showing comparisons between a spot size of the cathoderay tube of Embodiment 1 and the spot size of the cathode ray tube ofthe prior art.

FIG. 6 is a side section view of a glass tube member of a cathode raytube of Embodiment 2 according to the invention.

FIG. 7 is a side section view of an oxidation preventing apparatus inEmbodiment 2.

FIG. 8 is a side section view of a reduction apparatus by a reducing gasof Embodiment 2.

FIG. 9 is a partial section view showing an annular projection in awelding portion of a glass tube 44 and a metal part 45.

FIG. 10(a) is a partial section view showing relationships between innerdiameters of a metal part and a glass tube in Embodiment 2.

FIG. 10(b) is a partial section view showing a state in which the metalpart and the glass tube shown in (a) are welded together.

FIG. 11(a) is a partial section view showing a metal part and a glasstube in which an edge portion is chamfered.

FIG. 11(b) is a partial section view showing a state in which the metalpart and the glass tube of (a) are welded together.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

[Structure]

Hereinafter, Embodiment 1 of the invention will be described in detailwith reference to FIG. 1 to FIG. 5.

FIG. 1(a) shows an example in which the invention is applied to acathode ray tube for a projection-type TV.

An electron gun 2 of a cathode ray tube 1 of Embodiment 1 comprises aglass tube 13 which has a spiral high-resistor 23 functioning as a mainfocusing lens on an inner face. A G5 electrode 4 which is formed byworking a stainless steel plate, and a getter 5 are disposed in the leftend portion of the electron gun 2 which is closer to a fluorescentscreen 3. A G3 electrode 6, a G2 electrode 7, and a G1 electrode 8 aresupported by a multiform rod 9 in the right end portion of the electrongun 2. Illustration of the cathode is omitted.

As shown in FIG. 1(b), a ring-like elastic metal plate 11 is disposed onthe inner wall of the center portion of the above-mentioned glass tube13 so as to contact with the spiral high-resistor 23. One end of a leadwire 12 which passes through a hole 19 of the center portion of theglass tube is welded to the metal plate 11. The other end of the leadwire 12 is welded to an inner pin 10 of a stem 10A.

The glass tube 13 for the main focusing lens is produced by using L29F(an item number of a glass tube produced by Nippon Electric Glass Co.,Ltd.) which is of lead silicate glass used in a cathode ray tube of theprior art. L29F glass has a softening point of 620 deg. C., an annealingpoint (annealing temperature) of 435 deg. C., and a distortion point of395 deg. C. When the glass tube 13 is heat-treated at theabove-mentioned temperature or 450 deg. C., distortion of the glass tube13 can be eliminated because the temperature is higher than theannealing point of 435 deg. C. Since it is considerably lower than thesoftening point, the glass tube 13 is not deformed and the initial shapeis maintained. A seal ring 14 is bonded to each of both ends of theglass tube 13 by a frit. In order to prevent the seal rings from beingoxidized, preferably, the seal rings 14 are previously subjected todeposition of gold, gold plating, chromium plating, nickel plating, orthe like. The deposition of gold, the gold plating, the chromiumplating, or the nickel plating may be conducted after the glass tube 13and the seal rings 14 are connected to each other by a method by whichan oxide film is not formed on the surfaces. Preferably, a conductivefilm containing silver (Ag), palladium (Pd), ruthenium (Ru), or the likeis formed on the inner faces of both the end portions of the glass tube13. Preferably, the resistance of the film is not higher than onehundredth of the resistance of the spiral high-resistor 23. Theexistence of the film allows the electrical connection between thespiral high-resistor 23 and the seal rings 14 to be stably attained.Furthermore, since the potential difference of the connecting portionbetween the spiral high-resistor 23 and each of the seal rings 14decreases, distortion of the electric field is reduced, and deformationof the spot shape can be prevented.

L29F glass has the thermal expansion coefficient of 94×10⁻⁷ /deg. C. Asmaterials of the seal rings 14 and the frit, therefore, NS-1 (an itemnumber of a seal ring material produced by Sumitomo Special Metals Co.,Ltd.), and 7590 (an item number of a frit material produced by IwakiG1ass Co., Ltd.) which are listed in Table 1 were used. These materialshave a thermal expansion coefficient which is substantially equal tothat of L29F glass.

                  TABLE 1                                                         ______________________________________                                        Thermal expansion coefficients of materials used in                             Embodiment 1                                                                           Material name                                                                                Thermal expansion                                      Item number coefficient                                                      Part (Manufacturer) (30 to 380 deg. C.)                                     ______________________________________                                        Glass tube L29F (Nippon electric                                                                        94 × 10.sup.-7/deg. C.                           glass Co., Ltd.)                                                             Frit 7590 (Iwaki glass 99 × 10.sup.-7/deg. C.                            company, Limited)                                                            Seal ring NS-1 (Sumitomo special 94 × 10.sup.-7/deg. C.                  metals Co., Ltd.)                                                            High-resistor   -- 92 × 10.sup.-7/deg. C.                               member                                                                        material                                                                    ______________________________________                                    

[Production method]

Next, an embodiment of the method of producing a cathode ray tube of theinvention using the above-mentioned materials will be described withreference to a flowchart of FIG. 2.

FIG. 2 mainly shows steps of producing a main focusing lens in which thespiral high-resistor is formed on a glass tube.

FIG. 3(a) is a side section view of a main focusing lens 13A. The stepsof producing the main focusing lens 13A will be described hereafter. Thehole 19 is opened at the center portion of the glass tube 13 by using afile or the like (a hole opening step 31 in the flowchart of FIG. 2). Asthe method of opening the hole 19, other method such as ultrasonicmachining, laser machining, sandblasting, or the like may be employed.The glass tube 13 in which the hole 19 is opened is washed and dried (awashing and drying step 32 in the same), and thereafter the seal rings14 shown in FIG. 3(c) are thermally bonded to it's both ends by usingthe frit material 7590 shown in Table 1 (a frit baking step 33 in thesame). Thereafter, the washing and drying is again conducted (a washingand drying step 34 in the same).

A high-resistor paste 16 which will be described below is applied by thedip method to the glass tube to which the seal rings 14 are bonded inthis way (hereinafter, this is referred to as seal ring-attached glasstube 18) (a high-resistor applying step 35). The high- resistor paste 16is diluted with organic solvent such as toluene, acetone, or methylethyl ketone, so as to adjust the viscosity. As shown in FIG. 4, thehigh-resistor paste 16 is placed in a vessel 15. The vessel 15 iscommunicated with an application table 15A via a hose 17. The sealring-attached glass tube 18 is vertically fixed onto the applicationtable 15A, and a tube opening of the seal ring-attached glass tube 18 isclosely contacted with a filling hole 17A.

First, the vessel 15 is moved upward as indicated by the arrow A, then,the high-resistor paste 16 in it is poured into the seal ring-attachedglass tube 18 via the hose 17 by means of a siphonage. At this time, thelevel of the vessel 15 is adjusted so that the liquid level of thehigh-resistor paste 16 stops below the hole 19 of the seal ring-attachedglass tube 18. Next, the vessel 15 is lowered as indicated by the arrowB, and the high-resistor paste in the seal ring-attached glass tube 18is returned into the vessel 15. Immediately after this operation, hotair is blown in the state vertically holding the seal ring-attachedglass tube 18, whereby the high-resistor paste 16 is predried (apredrying and regular drying step 36).

A film thickness which is more uniform can be obtained by adequatelycombining the viscosity of the paste 16 and the lowering speed of thevessel 15, and by dissipating the solvent by the subsequent hot-airdrying. Next, the seal ring-attached glass tube 18 is verticallyinverted, and the high-resistor paste 16 is applied to the side lowerthan the hole 19 in the same method as described above. Thehigh-resistor film which is obtained in this way has a thickness of 5 to10 μm. Finally, the high-resistor paste is applied by a brush or thelike to a portion which is in the vicinity of the hole 19 and to whichno high-resistor paste adheres.

The high-resistor paste 16 is mainly a mixture of 0.5 to 5 wt. % of finepowder of ruthenium oxide and the remnant of powder of lead borosilicateglass having a thermal expansion coefficient of 90 to 100×10⁻⁷ /deg. C.A small amount of an additive of a metal oxide or an organic metal, andan organic binder are added to the mixture.

If an example is shown, powder of ruthenium oxide of a mean particlediameter of 0.3 μm and powder of glass (glass composition is PbO 77 wt.%; B₂ O₃ 18 wt. %; and SiO₂ 5 wt. %) of a mean particle diameter of 1.5μm are mixed with each other at the weight ratio of 3 to 97. Terpineolinto which 10% of ethyl cellulose is dissolved as the organic binder,and a small amount of copper oxide are added to the mixture. Thismixture is kneaded by a three-roll kneader, thereby producing thehigh-resistor paste 16.

The combination of the high-resistor materials of the high-resistorpaste 16 was conducted in consideration of the following three points.

As a first point, the thermal expansion coefficient of the high-resistormaterial was made coincident with the thermal expansion coefficient ofthe materials of the glass tube 13, the metal plate 11, and the sealrings 14 of the other three parts. Thereby, it is possible to prevent acrack and separation due to the heat treatment from occurring in thehigh-resistor film. Furthermore, it is possible to prevent the glasstube 13 and the seal rings 14 from being separated from each other.Table 1 shows an example of the thermal expansion coefficients of thematerials use in the embodiment. In a practical use, the glass tube 13,the frit material, the seal rings 14, and the high-resistor materialconstituting the electron gun 2 have a thermal expansion coefficient inthe range of 85 to 105×10⁻⁷ /deg. C.

As a second point, in order to eliminate distortion of the glass tube13, the softening point of the glass powder of the high-resistormaterial is selected so as to be lower than the annealing point of theglass tube 13. In the embodiment, since the annealing point of the glasstube 13 is 435 deg. C., the softening point is set to be lower than itor 430 deg. C. In order that ruthenium oxide of the conductive fineparticles and the glass powder which is a nonconductive material meltand the ruthenium oxide uniformly enters the glass film to form ahigh-resistor, the high-resistor material must be fired at a temperaturehigher than softening point. Therefore, 450 deg. C. is suitable as thefiring temperature.

The above-mentioned first and second points were realized mainly bysetting the composition ratio of the glass powder to PbO 77 wt. %; B₂ O₃18 wt. %; and SiO₂ 5 wt. % as described above.

As a third point, the size in the cutting of the applied high-resistormaterial into a spiral shape is selected to be an appropriate value. Asdescribed above, the mixing ratio of ruthenium oxide to the glass powderwas determined to be 3 to 97 so that the objective resistance isattained when the firing is conducted at a firing temperature of 450deg. C. and a firing time of 10 minutes.

Next, the seal ring-attached glass tube 18 to which the high-resistorpaste 16 is applied is subjected to the regular drying at about 250 deg.C. (the predrying and regular drying step 36). Then, the sealring-attached glass tube 18 is rapidly rotated about the axis by alathe, and the high-resistor film 22 applied to the inner face of theseal ring-attached glass tube 18 is cut by a cemented carbide tool intoa spiral shape to form cutting regions 20 (a spirally cutting step 37).

In the cutting, as shown in FIG. 3(a), five spiral cutting regions 20are disposed in each of the left and right halves so as to besymmetrical about the hole 19 of the glass tube 13. In FIG. 3(a), thespiral cutting regions 20 and non-cut portions are alternatinglyarranged. The cutting width and the pitch may be changed as required. Anexample is shown in FIG. 3(b) which is an enlarged view of the spiralcutting regions 20. In FIG. 3(b), portions in which the high-resistorfilm is removed away by the cutting are indicated by grooves 21. In bothFIG. 3(a) and FIG. 3(b), the high-resistor film 22 is indicated byoblique lines of the same direction. Incidentally, the spiral grooves 21may be cut at the equal pitches in the axial direction of the glass tube13.

The grooves 21 of the spiral high-resistors 23 shown in FIG. 1 areevenly cut in the axial direction of the glass tube 13 except the centerportion of the glass tube 13. In FIG. 1, the grooves 21 after cuttingare indicated by oblique solid lines in the same manner as the grooves21 shown in FIG. 3(a), and, the spiral high-resistor 23 exists in theblank portions. Therefore, the spiral pattern shown in FIG. 1 isdifferent from the spiral pattern which is shown in FIG. 3(a) and whichis unevenly formed in the axial direction of the glass tube 13.

The glass tube 13 on which the spiral grooves 21 are formed isvertically disposed and is then fired at 450 deg. C. for 10 minutes (afiring step 38). After the firing process, the metal plate 11 isattached to the inner wall of the hole 19 of the glass tube 13.Incidentally, as shown in FIG. 1(c), in place of the metal plate 11, apin 11A may be inserted into the hole 19 and, before the firing process,the high-resistor paste may be applied so as to cover the pin 11A andthe high-resistor film 22.

By the above-mentioned firing process, the organic binder decomposes andburns. The high-resistor film 22 is vitrified and fixedly bonded in theseal ring-attached glass tube 18, so as to constitute the spiralhigh-resistor 23. The resistor has a thickness of 3 to 6 μm. Theabove-mentioned firing process causes the spiral high-resistor film 23to be electrically connected to the respective seal rings 14 in both theend portions of the glass tube 13. In the center portion of the glasstube 13, moreover, both the right and left portions of the spiralhigh-resistor 23 are electrically connected to the metal plate 11.

The properties of the spiral high-resistor film 23 which was formed inaccordance with the invention are shown in Table 2. The case which wasformed by using ruthenium hydroxide is shown as a conventional example.As apparent from Table 2, in the case of the invention, the use of apaste-like high-resistor material allows the film thickness to beincreased. As a result, the resistance is 20 GΩ or lower than that ofthe conventional example, and the dispersion of the resistance can belargely reduced.

                  TABLE 2                                                         ______________________________________                                        Properties of spiral high-resistance of the conventional                        example and the invention                                                             Firing   Film             Dispersion                                  tempera- thick-  of                                                           ture ness Resistance resistance                                             ______________________________________                                        Conventional                                                                            450      1.3 μm 26 GΩ                                                                          ±70%                                     example deg. C.                                                               Embodiment 450 4.5 μm 20 GΩ ±20%                                   deg. C.                                                                    ______________________________________                                    

The spiral high-resistor 23 functioning as the main focusing lens of thecathode ray tube 1 is to give a potential distribution between the anodevoltage and the focus voltage, and must have a high resistance in orderto make little current flow.

The target value of the resistance of the spiral high-resistor 23 is setto be about 20 GΩ under the state cut into a spiral shape as shown inFIG. 3(a). When the positive anode voltage of 32 KV is applied to theseal rings 14 of both ends of the glass tube 13 of FIG. 1 and thepositive focus voltage of 7 kV is applied to the metal plate 11, forexample, a leak current (Ig4) flowing through the spiral high-resistor23 is about (32 KV-7 kV)/20 GΩ=1.25 μA. Since the current value becomessuch as this, the operation of the cathode ray tube 1 is notsubstantially affected and a stable operation is ensured.

The resistance of the spiral high-resistor 23 must be within apredetermined range. In other words, when the resistance becomes toohigh, for instance 1 TΩ, almost no leak current flows, and therefore arequired potential distribution cannot be obtained and the potentialbecomes unstable owing to the dielectric function of the glass.According to experiments conducted by the inventor et al., 100 GΩ orlower is preferable. In the above-mentioned case where the anode voltageis 32 KV and the focus voltage is 7 kV, a leak current (Ig4) of 0.25 μAor larger must flow.

Conversely, when the resistance is too low, a large leak current (Ig4)flows, therefore, a potential difference is produced across resistors(not shown) which are connected in series to an electrode (in theembodiment, G4) for supplying the focus voltage in order to form themain focusing lens, thereby changing the focus voltage. When the leakcurrent (Ig4) is changed, particularly, the focus voltage is changed. Inorder to prevent such a change from occurring, the leak current (Ig4)must be 30 μA or less and the resistance must be 0.8 GΩ or higher.

Next, as shown in FIG. 1 the parts from the G3 electrode 6 to the G1electrode 8 which have been preliminarily assembled on the multiform rod9 are welded to the right side seal ring 14, and the parts from the G5electrode 4 to the getter 5 are welded to the left side seal ring 14,thereby completing the electron gun 2 (an electron gun assembling step39). Next, the electron gun 2 is enclosed into the completed bulb inwhich the fluorescent screen and the like are disposed. The subsequentsteps are identical with the production method of a conventional cathoderay tube and hence their description is omitted (a cathode ray tubeproducing step 40).

[Comparison of performance between Embodiment 1 and conventionalexample]

FIG. 5 shows a spot size of the cathode ray tube of the invention and aspot size of a conventional one. The abscissa indicates the anodecurrent and the ordinate indicates the spot size. The spot sizesindicated by the solid line of the cathode ray tube obtained by theproduction method of the invention can be made smaller than the spotsizes indicated by the broken line of the conventional one over therange of the anode current from the small current region to the largecurrent region. Accordingly, compared with a conventional cathode raytube in which the main focusing lens is formed by a combination of metalelectrodes, very good resolution properties can be obtained.

Incidentally, in the above-mentioned embodiment, a unipotential (UPF)type electron gun in which symmetric voltages are supplied across theconnecting point disposed in the center hole 19 of the glass tube 13 andtwo connecting points closer to both the ends of the glass tube has beenshown. The invention may be applied also to an electron gun of anothertype, or a bipotential (BPF) type electron gun. In this case, theconnecting point disposed in the center hole 19 of the glass tube 13 isnot required and the anode voltage and focus voltages are respectivelysupplied to metal electrodes connected to both the ends of the glasstube 13.

Embodiment 2

Embodiment 2 will be described with reference to FIG. 6 to FIG. 11.

Embodiment 2 relates to a glass tube assembly in which a spiralhigh-resistor functioning as a main focusing lens of an electron gun ofa cathode ray tube is disposed.

[Configuration of the glass tube assembly]

FIG. 6 is a side section view of a glass tube assembly 43. The glasstube assembly 43 is configured of a glass tube 44, and seal rings 45which are metal parts attached to both ends of the glass tube 44. Theglass tube 44 is substantially identical in shape with the glass tube 13of Embodiment 1, but different in material therefrom. Also the sealrings 45 are different in material from the seal rings 14 of Embodiment1.

As the material of the glass tube 44, BCL which is borosilicate glass orSKC which is soda glass is used. BCL and SKC are product names of therespective manufacturers and their composition and properties are shownin Table 4 (sic). As shown in Table 3, BCL and SKC are lower in volumeresistivity than L-29F.

                  TABLE 3                                                         ______________________________________                                                      Name of glass tube material                                     Composition and properties                                                                    L-29F     BCL     SKC                                         ______________________________________                                        PbO (wt. %)     28.0      0       0                                             SiO.sub.2 (wt. %) 60.0 72.0 70.3                                              Al.sub.2 O.sub.3 (wt. %) 1.0 7.0 2.0                                          B.sub.2 O.sub.3 (wt. %) 0 10.5 1.2                                            MgO (wt. %) 0 0 2.8                                                           CaO (wt. %) 0 0.5-1.0 5.9                                                     BaO (wt. %) 0 1.5-2.0 0                                                       Na.sub.2 O (wt. %) 8.0 7.5 16.0                                               K.sub.2 O (wt. %) 3.0 7.5 1.3                                                 Softening point (deg. C.) 615 785 694                                         Annealing point (deg. C.) 435 570 525                                         Thermal expansion 94 52 98.5                                                  coefficient × 10.sup.-7 /deg. C.                                        Volume resistivity 13.4 11.1 10.4                                             logρ: Ωcm (100 deg. C.)                                           ______________________________________                                    

When a current flows through the spiral high-resistor 20 formed on theinner face of the glass tube 13 as shown in FIG. 3(b), charges areaccumulated in the bottoms of the grooves 21, i.e., the surface of theglass tube. The amount of accumulated charges varies in accordance withthe volume resistivity of the glass, and as the volume resistivity issmaller the amount is smaller. The inventor has found that chargesaccumulated in the bottoms of the grooves 21 influence the operation ofthe main focusing lens, and this influence causes the shape of a spot onthe display screen to be irregularly varied. Therefore, it is preferableto reduce charges accumulated in the bottoms of the grooves 21 to anamount as small as possible. In the embodiment, since the glass tube 44is produced by using BCL or SKC which is a glass material having arelatively low volume resistivity, charges accumulated in the glasssurface of the grooves 20 can be reduced and the above-mentionedirregular variation of the spot shape can be suppressed.

In the case where BCL is used as the material of the glass tube 44, ahigh-resistor paste which is applied to the inner face of the glass tube44 will be described hereafter. Since the thermal expansion coefficientof BCL is 52×10⁻⁷ /deg. C. as shown in Table 4 (sic), resistor materials3, 4, and 5 which have a thermal expansion coefficient of 55 to 60×10⁻⁷/deg. C. are suitable as the resistor material which is used in thehigh-resistor paste. It is possible to prevent the high-resistor filmfrom being peeled off from the glass tube 44 by using materials of eachother thermal expansion coefficients in the glass tube and thehigh-resistor. In the case where a material into which a filler is addedis used as the resistor material, since the homogeneity of thedispersant film is slightly inferior, it is preferable to use a resistormaterial 3 which contains a smaller amount of lead. In this case, sinceglass particles have a softening point as high as 515 deg. C., thefiring temperature must be raised to about 520 to 550 deg. C.

                  TABLE 4                                                         ______________________________________                                                   Name of resistor material                                                       Resistor                                                                              Resistor                                                                              Resistor                                                                            Resistor                                                                            Resistor                               Composition and prop- material material material material material                                                    erties 1 2 3 4 5                    ______________________________________                                        RuO.sub.2 (wt. %)                                                                          8       3       10    10    8                                      PbO (wt. %) 66 74.5 36 48 52                                                  B.sub.2 O.sub.3 (wt. %) 16 17.5 15 11 12                                      SiO.sub.2 (wt. %) 7 5 5.5 1.5 2                                               ZnO (wt. %) 2.8 0 31.5 3 3                                                    Al.sub.2 O.sub.3 (wt. %) 0.2 0 1 2 2                                          SnO.sub.2 (wt. %) 0 0 1 0 0                                                   ZrO.sub.2 0 0 0 16.5 14                                                       filler (wt. %)                                                                SiO.sub.2 0 0 0 8 7                                                           filler (wt. %)                                                                Al.sub.2 O.sub.3 0 0 0 0 0                                                    filler (wt. %)                                                                Softening point 490 430 515 430 430                                           (deg. C.)                                                                     Annealing point 395 365 440 365 365                                           (deg. C.)                                                                     Thermal expansion 80 90 60 55 55                                              coefficient ×                                                           10.sup.-7 /deg. C.                                                          ______________________________________                                    

[Welding of the glass tube and the metal parts]

As shown in FIG. 6, the seal rings 45 as metal parts are attached toboth ends of the glass tube 44. The glass tube 44 and the seal rings 45are welded to each other by using a frit or are welded by melting theglass tube 44 itself. The glass tube 44 and the seal rings 45 must beproduced by using materials of each other close thermal expansioncoefficients. In the case where the glass tube 44 is made of BCL, sincethe thermal expansion coefficient is 52×10⁻⁷ /deg. C., KV-2, KV-15,YEF-29-7, DK, or the like which has a thermal expansion coefficient of44 to 55×10⁻⁷ /deg. C. as shown in Table 5 is used as the metal materialof the seal rings 45. In the case where the glass tube 44 is made ofSKC., since the thermal expansion coefficient is 98.5×10⁻⁷ /deg. C.,NS-1 which has a thermal expansion coefficient of 94 to 100 is used asthe metal material of the seal rings 45. When the glass tube 44 and theseal rings 45 are welded by a frit material, the thermal expansioncoefficient of the frit material must be coincident with the thermalexpansion coefficient of the glass tube 44. From the points mentionedabove, materials having a thermal expansion coefficient in the range of36 to 105×10⁻⁷ /deg. C. are used for the glass tube 44, the frit, theseal rings 45, and the high-resistor which is applied to the inner faceof the glass tube 44 in a later step.

                  TABLE 5                                                         ______________________________________                                        Manufacturers                                                                                              Hitachi Nippon                                     Name of Sumitomo Special Metals Mining                                        metal Metals Co., LTD. LTD. Co., LTD.                                       material                                                                              NS-1    KV-2    KV-15  YEF-29-17                                                                             DK                                     ______________________________________                                        Thermal 94-100  44-52   46     48      44-55                                    expansion                                                                     coefficient ×                                                           10.sup.-7 /                                                                   deg. C.                                                                       (30-400                                                                       deg. C.)                                                                    ______________________________________                                    

The glass tube 44 and the seal rings 45 are welded by heating them atthe temperature of 800 deg. When this heating is conducted in the air,an oxide film is formed on the surfaces of the seal rings 45. It isknown that the oxide film enhances the bond strength between glass andmetal. In this respect, therefore, the existence of the oxide film ispreferable. On the other hand, when the high-resistor film was formed inthe glass tube 44 in the later step, there is a case where theelectrical connection between the high-resistor film and the seal rings45 becomes incomplete. In the embodiment, the following variouscountermeasures are taken in order to prevent an oxide film from beingformed on the surfaces of the seal rings 45.

[Method of suppressing oxidation of the seal rings]

In the method, immediately after the seal rings 45 are welded to theglass tube 44 in the air, the glass tube 44 having the seal rings 45 isplaced in a reducing gas atmosphere such as hydrogen or an inert gasatmosphere such as nitrogen. By means of this process, the formation ofthe oxide film on the surfaces of the seal rings 45 can be suppressed.This process may be applied also to the metal plate 11 shown in FIG. 1functioning as the focusing voltage supply portion in a similar manner.

[Method and apparatus for preventing the seal rings from being oxidized]

Although it has been described in Embodiment 1, that, as acountermeasure for preventing the seal rings from being oxidized, asurface treatment such as deposition of gold, gold plating, chromiumplating, or nickel plating is previously conducted, in the embodiment,the objective is to prevent oxidation from occurring, without conductingsuch a previous surface treatment.

(Oxidation prevention by means of an inert gas)

FIG. 7 is a section view showing main portions of a welding apparatusfor welding the glass tube 44 and the seal ring 45 together in anatmosphere of an inert gas such as nitrogen gas. In the weldingapparatus, the glass tube 44 is welded to the seal ring 45 by meltingthe glass tube itself and without using a frit. In the figure, ashielding tube 51 is the tube of a heat resistant insulator which has ablowing hole 52 for introducing nitrogen gas in the upper portion. Theinner diameter of the shielding tube 51 is made larger than the outerdiameters of the glass tube 44 and the seal ring 45 by a predeterminedsize. The glass tube 44 and the seal ring 45 are inserted into theshielding tube 51 by a jig which is not shown, by maintaining apredetermined positional relationship. A carbon block 53 having a hole53A in the center portion is placed on the seal ring 45, and a metalblock 54 which has a hole 54A in the center portion and which functionsas a weight is placed on the carbon block 53. The carbon block 53 ismade of carbon because melting glass does not adhere with carbon. Theouter diameters of the carbon block 53 and the metal block 54 are madesmaller than the inner diameter of the shielding tube 51.

A heating coil 55 for conducting high frequency induction heating isdisposed in the vicinity of the seal ring 45 on the outer periphery ofthe shielding tube 51. Three or six restriction rods 58 made of ceramicsare disposed in the shielding tube 51 in order to restrict a meltinglength of the glass tube 44 during the welding process (hereinafter isreferred to as melting margin). During a period when the glass tube 44and the seal rings 45 are welded, the upper end of the glass tube 44melts and the length of the tube is reduced, so that the seal ring 45 islowered by the gravity of the metal block 54. When the seal ring 45 islowered by a predetermined distance, the lower face abuts against theupper ends of the restriction rods 58, thereby inhibiting furtherlowering. As a result, the melting margin of the glass tube can berestricted to a predetermined value (0.2 to 0.3 mm).

Next, the operation of the welding apparatus will be described. Asindicated by an arrow 56, an inert gas such as nitrogen gas isintroduced through the blowing hole 52 of the shielding tube 51. Theinert gas enters the interior space of the glass tube 44 through therespective holes 54A and 53A of the metal block 54 and the carbon block53. Furthermore, the inert gas enters the space between the outer wallof the glass tube 44 and the inner wall of the shielding tube 51,through a gap between the inner wall of the shielding tube 51 and theouter peripheral faces of the metal block 54, the carbon block 53, andthe seal ring 45. Since the inert gas flows through the spaces insideand outside the glass tube 44, the seal ring 45 is placed in the inertgas atmosphere so as not to be contacted with oxygen.

Subsequently, a high frequency current is supplied to the heating coil55 to heat the seal ring 45, and the end portion of the glass tube 44contacted with the seal ring 45 melts and both are bonded. During theabove-mentioned heating process, since the seal ring 45 is in the inertgas atmosphere, the surface is not oxidized.

In the welding apparatus shown in FIG. 7, the seal ring 45 and the endregion of the glass tube 44 contacted therewith is mainly heated bylocal heating by the high frequency induction heating. Therefore, thetemperature of the glass tube 44 as a whole is not highly raised andhence it is possible to prevent the glass tube 44 from being thermallydeformed. Since a softening point of the material of the glass tube 44is 785 deg. C., in place of using the above-mentioned reductionapparatus, when the whole of the glass tube 44 is placed in an oven of atemperature of 800 deg. C. or higher and a welding process is conducted,the whole of the glass tube 44 is liable to be deformed. In the weldingapparatus of the embodiment, only the portion corresponding to the sealring 45 is heated and hence deformation of the whole of the glass tube44 does not occur.

[Method and apparatus for removing an oxide film of the seal ring]

In the case that a welding process is conducted without depending on theabove-described oxidation preventing method, description is made as to amethod and an apparatus for removing an oxide film formed on the surfaceof the seal ring 45 hereafter. When the glass tube 44 and the seal ring45 are welded together by a frit, or when a frit is not used and theglass tube 44 is welded to the seal ring 45 by melting the glass tubeitself, if they are heated in the air, the surface of the heated sealring is oxidized and an oxide film is formed. When an oxide film isformed on the surface of the seal ring 45, the electrical connectionbetween the high-resistor film and the seal ring 45 becomes unstable dueto the oxide film. With respect to this point, there arises a similarproblem also when the metal plate 11 of the focusing voltage supplyportion is attached before the application of the high-resistor paste.

(Apparatus for removing an oxide film by a reducing gas)

FIG. 8 shows a reduction apparatus for reducing the seal ring 45 havingan oxide film in a reducing gas. In the figure, a shielding tube 61 ofthe reduction apparatus is a tube which has heat resistance, and has ablowing hole 63 for the reducing gas at an upper end part of the figure.A straightening mesh 62 which is formed by a wire net or the like isdisposed in the tube which is separated from the blowing hole 63 by apredetermined distance so as to be perpendicular to the center axis ofthe tube. The glass tube 44 to which the seal ring 45 is welded isinserted to a position which is separated from the straightening mesh 62by a predetermined distance. As indicated by an arrow 64, hydrogen gasserving as a reducing gas is introduced through the blowing hole 63. Theintroduced hydrogen gas is straightened by the straightening mesh 62, sothat the distribution of the flow rate becomes uniform in the shieldingtube 61 after passing through the straightening mesh 62.

The reducing operation by the apparatus is conducted as follows. Afterthe seal ring 45 is welded to the glass tube 44 by heating in the air,the glass tube 44 having the seal ring is inserted into the shieldingtube 61 under the state where the temperature of the seal ring 45 ishigh. In the shielding tube 61, the hydrogen gas contacts with the sealring 45 of a high temperature and the oxide on the surface of the sealring 45 is reduced by the hydrogen gas. Since the flow rate of thehydrogen gas is uniformalized by the straightening mesh 62, the surfaceof the seal ring 45 is reduced uniformly.

In the reduction apparatus by the hydrogen gas shown in FIG. 8, when thehydrogen gas blown from a lower opening 61A of the shielding tube 61 isignited, flame 61B is formed and the hydrogen gas burns. The inventorhas found that, when hydrogen gas burns in this way, it is possible toprevent the air from entering the shielding tube 61 through the opening61A at the lower end of the shielding tube 61. Since the entrance of theair is inhibited, the seal ring 45 is reduced more satisfactorily anduniformly. Although, an oxide film is formed on the seal ring 45 as aresult of the heating in the air, the oxide film has a function ofenhancing the bonding strength between the seal ring 45 and the glasstube 44. Therefore, when the welding process is conducted in the airand, after the welding, the surface oxide film of the seal ring 45 isreduced by hydrogen gas and removed away, it is possible to attain ahigh bonding strength and also to prevent a contact failure between theseal ring 45 and the high-resistor film 22 from occurring when thehigh-resistor film 22 is formed in a later step.

(Removal of an oxide film by a chemical process)

Next, a method of removing an oxide film of the seal ring 45 by achemical process will be described. Chemical which is used ishydrochloric acid or a hydrochloric acid rust removing agent.Commercially available products of such chemicals include a liquid rustpreventing agent which has a product name of JASCO RS-207 and isproduced by JAPAN SURFACE COMPANY.

In the case that JASCO RS-207 is used, it is immersed in the liquid of apredetermined concentration and temperature for several to ten-oddminutes. Then, an oxide film which raises to the surface of the sealring 45 is rubbed off with cloth or the like and then washed away bywater. In order to prevent an oxide film from being again formed, it isimmersed in an alkaline neutralizing rust preventing agent. As theneutralizing rust preventing agent, for example, JASCO M-194 which isproduced by the same manufacturer as that of JASCO RS-207 is used.

(Method of mechanically removing oxide film)

Next, a method of mechanically shaving an oxide film of the seal rings45 will be described. In an example of such a mechanical method, theoxide film is shaved off by rotating a metal brush. In this process,there may arise a case where rust preventing oil of the metal brush, ashaved-off oxide film of a high temperature, metal powder, and the likescatter, adhere to the glass tube 44 and are baken. In order to preventsuch a case from occurring, it is preferable to conduct the process byspraying water to the glass tube 44.

[Shape of end portion of glass tube 44]

When the glass tube 44 and the seal ring 45 were welded together, theend portion of the glass tube 44 may be deformed and upheave as shown inFIG. 9 so as to form an annular projection 71. When the melting marginof the glass tube 44 is 0.2 to 0.3 mm, the annular projection 71 isformed. In the case that the annular projection 71 is formed, when thehigh-resistor paste is applied in a later step, the thickness of theapplied layer of the high-resistor paste is reduced on the portion ofthe annular projection 71 and a conduction failure is liable to occur.The annular projection 71 can be removed away by mechanically shavingoff after the welding process. However, this process is cumbersome. Inorder to prevent an annular projection from being formed, therefore, theglass tube 44 and the seal rings 45 are configured as shown below.

FIG. 10(a) is a section view of the glass tube 44 and the seal ring 45which shows a first configuration. In the figure, the inner diameter ofthe seal ring 45 is made smaller than the inner diameter of the glasstube 44. When the glass tube 44 and the seal ring 45 which are thusconfigured are welded together, the end portion of the glass tube 44 hasa smooth flare shape and an annular projection is not formed as shown inFIG. 10(b). When the difference between the inner diameters of the sealring 45 and the glass tube 44 is appropriately set, a desired flareshape can be formed.

FIG. 11(a) is a section view of the glass tube 44 and the seal ring 45which shows a second configuration. In the figure, the inner diameter ofthe seal ring 45 is substantially equal to the inner diameter of theglass tube 44. The inner face of the end portion of the glass tube 44 ischamfered so as to form a chamfered portion 72. For example, the size ofchamfering is 0.2 to 0.5 mm. When the glass tube 44 which is thusconfigured is welded, the inner diameter of the glass tube 44 becomessubstantially equal to the inner diameter of the seal ring 45 and anannular projection is not formed as shown in FIG. 11(b).

Industrial Applicability

The method of producing a cathode ray tube of the invention comprisesthe steps of: applying a high-resistor paste on the inner face of aglass tube for the main focusing lens and then drying the paste, therebyforming a high-resistor film; forming a spiral groove in theabove-mentioned high-resistor film; and firing the above-mentionedhigh-resistor film at 420 to 550 deg. C. As the above-mentionedhigh-resistor paste, what provides the above-mentioned spiralhigh-resistor with a resistance that is not lower than 0.8 GΩ and nothigher than 100 GΩ by firing in the above-mentioned temperature range isused. According to this matter, it is not required to use expensiveceramic or quartz glass as the material of the glass tube for the mainfocusing lens, and usual inexpensive glass can be used.

A high-resistance suspension containing a mixture of ruthenium hydroxideand glass powder which is problematic in the prior art is not used, anda high-resistor material in which ruthenium oxide is added to glasspowder is used as a high-resistor paste, whereby the temperaturedependency of the resistance of the spiral high-resistor 23 can bereduced.

Since glass powder having a softening point which is lower than theannealing point of the glass tube is used, the firing temperature islowered, so that an inexpensive glass tube can be used. Furthermore, thefiring at a low temperature causes distortion of the glass tube to beeliminated and can prevent the glass tube from being deformed.

Since the thermal expansion coefficients of the glass tube, the fritmaterial, the seal ring, and the high-resistor material which constitutethe electron gun 2 are made coincident with each other, it is possibleto prevent a crack and separation from occurring in the high-resistorfilm, and also to prevent the seal ring from being separated from theglass tube.

Since the spiral high-resistor is provided with a resistance that is notlower than 0.8 GΩ and not higher than 100 GΩ, a potential distributionsuitable for an electron beam is obtained by a voltage applied to thespiral high-resistor disposed on the inner face of the glass tube.

In the main focusing lens using the high-resistor, since the potentialgradient forming an electron lens is gentle and uniform, an effect inwhich the lens diameter of the main focusing lens is apparentlyincreased is produced. As a result, the spherical aberration can bereduced and a high-resolution cathode ray tube can be realized.

Since an oxide film of the seal ring is removed, it is possible torealize a good connection state between the metal part and thehigh-resistor film.

Since the welding process is conducted in an inert gas atmosphere, thesurface of the seal ring can be prevented from being oxidized. Since anoxide film is removed, moreover, it is possible to prevent a connectionfailure between the seal ring and the high-resistor film from occurringwhen the high-resistor film is formed.

Since the inner diameter of the seal ring is made smaller than the innerdiameter of the glass tube, it is possible to prevent an annularprojection from being formed during the welding process. Since the innerface of the end portion of the glass tube is chamfered, moreover, it issimilarly possible to prevent the annular projection from being formed.

Although the invention has been described in its preferred forms with acertain degree of particularity, it is understood that the presentdisclosure of the preferred forms should be changed in the details ofconstruction, and variations of the combination and arrangement ofelements may be realized without departing from the spirit and the scopeof the invention as claimed.

We claim:
 1. A method of producing a cathode ray tube having a spiralhigh-resistor formed on an inner face of a glass tube which functions asa main focusing lens for an electron gun, wherein the method ofproducing the cathode ray tube comprises the steps of:applying ahigh-resistor paste on the inner face of the glass tube, thereby forminga high-resistor film; forming a spiral groove in said high-resistorfilm; and firing said high-resistor film having said spiral groove at atemperature in the range of 440 to 460 deg. C. to obtain the spiralhigh-resistor, whereby the spiral high-resistor has a resistance valuethat is not lower than 0.8 GΩ and not higher than 100 GΩ as a result ofthe firing in said temperature range.
 2. A method of producing a cathoderay tube according to claim 1,wherein the high-resistor paste comprisesruthenium oxide added to glass powder, the glass powder having asoftening temperature that is lower than an annealing temperature of theglass tube.
 3. A method of producing a cathode ray tube according toclaim 2,wherein said glass powder contains about 77 wt. % of PbO, about18 wt. % of B₂ O₃, and about 5 wt. % of SiO₂, and the ruthenium oxide ofsaid high-resistor paste is in the range of 0.5 to 5 wt. %.
 4. A methodof producing a cathode ray tube according to claim 1,wherein the thermalexpansion coefficient of the glass tube and the high-resistor materialis in the range of 85 to 105×10⁻⁷ /deg. C.
 5. A method of producing acathode ray tube according to claim 1,wherein the resistance of saidspiral high-resistor is set so that, when a difference voltage betweenan anode voltage and a focus voltage is applied to said spiralhigh-resistor, a current flowing through said spiral high-resistor is inthe range of 0.25 μA to 30 μA.
 6. A cathode ray tube having a spiralhigh-resistor formed on an inner face of a glass tube having a sealring, and serving as a main focusing lens for an electron gun, whereinthe cathode ray tube comprises:a high-resistor paste for forming saidspiral high-resistor, the paste comprising ruthenium oxide and glasspowder, the glass powder having a softening temperature that is lowerthan an annealing temperature of said glass tube.
 7. A cathode ray tubeaccording to claim 6, wherein the thermal expansion coefficients of theglass tube, the seal ring, and the high-resistance material whichconstitute the electron gun are in the range of 85 to 105×10⁻⁷ /deg. C.8. A cathode ray tube according to claim 6, wherein the resistance ofsaid spiral high-resistor is selected so that, when a difference voltagebetween an anode voltage and a focus voltage is applied to said spiralhigh-resistor, a current flowing through said spiral high-resistor is inthe range of 0.25 μA to 30 μA.
 9. A method of producing a cathode raytube, the tube having a main focusing lens in which a high-resistor isformed into a spiral shape by a high-resistance material having athermal expansion coefficient of 36 to 105×10⁻⁷ /deg. C., on an innerface of a glass tube having a tubular structure made of borosilicateglass or soda glass material having a thermal expansion coefficient of36 to 105×10⁻⁷ /deg. C. and a volume resistance of 1×10¹⁰ to 1×10¹²ohm·cm, comprising the steps of:disposing metal terminals to both endsof said glass tube, respectively, said metal terminals providing forelectrical connection with electrodes of an electron gun; applying ahigh-resistor paste to said glass tube, thereby forming a high-resistorfilm; forming said high-resistor film into a spiral structure; firingsaid glass tube at a temperature in the range of 420 to 550 deg. C.; andconnecting said metal terminals of the glass tube to electrodes wherebythe electron gun is formed.
 10. A method of producing a cathode ray tubeaccording to claim 9,further comprising the step of disposing a focusingvoltage supply portion in the vicinity of the center of said glass tube.11. A method of producing a cathode ray tube according to claim9,wherein the high-resistor paste is comprised of ruthenium oxide addedto a glass material having a softening temperature that is lower than anannealing temperature of said glass tube.
 12. A method of producing acathode ray tube according to claim 9,wherein the resistance of thespiral high-resistor is set so that, when a difference voltage betweenan anode voltage and a focus voltage is applied to said spiralhigh-resistor, a current flowing through said spiral high resistor is inthe range of 0.25 μA to 30 μA.
 13. A method of producing a cathode raytube according to claim 11,wherein said glass material contains in therange of 25 to 40 wt. % of a filler material comprising at least one ofZrO₂, SiO₂, and A1₂ O₃.
 14. A method of producing a cathode ray tubehaving a spiral high-resistor formed on an inner face of a glass tube,the spiral high-resistor functioning as a main focusing lens for anelectron gun, wherein the method of producing the cathode ray tubecomprises the steps of:bonding a metal terminal to said glass tube,melting a frit, wherein the thermal expansion coefficients of said glasstube, said frit, said metal, terminal and a material of said spiralhigh-resistor is in the range of 36 to 105×10⁻⁷ /deg. C.
 15. A method ofproducing a cathode ray tube having a spiral high-resistor formed on aninner face of a glass tube, which functions as a main focusing lens foran electron gun, wherein, the method of producing the cathode ray tubecomprises the steps of:welding said glass tube to a metal terminal bymelting a frit or said glass tube itself, in order to bond the metalterminal to said glass tube, said glass tube and said metal part beingconducted in a reducing gas atmosphere or an inert gas atmosphere.
 16. Amethod of producing a cathode ray tube having a spiral high-resistorformed on an inner face of a glass tube which functions as a mainfocusing lens for an electron gun, wherein, the method of producing thecathode ray tube comprises the steps of:welding said glass tube to ametal terminal by melting a frit or said glass tube itself, in order tobond the metal terminal to said glass tube, the metal terminal having afilm for preventing oxidation on said metal terminal.
 17. A method ofproducing a cathode ray tube according to claim 16, wherein said filmfor preventing oxidation is a film which is formed by one of depositionof gold, gold plating, chrome plating, and nickel plating.
 18. A methodof producing a cathode ray tube having a spiral high-resistor formed onan inner face of a glass tube which functions as a main focusing lensfor an electron gun, the method of producing the cathode ray tubecomprising the steps of:welding said glass tube to the metal terminal bymelting a frit or said glass tube itself; and removing an oxide film ona surface of said metal terminal after welding said glass tube to saidmetal terminal whereby the metal terminal electrically connects to thespiral high-resistor for connection as the focusing lens of the electrongun.
 19. A method of producing a cathode ray tube according to claim 18,wherein said step of removing the oxide film is a reduction step bymeans of heating in a hydrogen or hydrogen-mixture gas atmosphere.
 20. Amethod of producing a cathode ray tube according to claim 19, whereinthe hydrogen or hydrogen-mixture gas atmosphere is formed by passinghydrogen or a hydrogen-mixture gas through a straightening mesh.
 21. Amethod of producing a cathode ray tube according to claim 19, whereinthe hydrogen or hydrogen mixture gas atmosphere prevents entrance ofoxygen from occurring by burning hydrogen.
 22. A method of producing acathode ray tube according to claim 18, wherein said step of removingthe oxide film comprises the step of immersing said metal terminal inhydrochloric acid or a hydrochloric acid rust removing agent.
 23. Amethod of producing a cathode ray tube according to claim 22, whereinsaid step of removing the oxide film further comprises the step of,after immersing said metal terminal in hydrochloric acid or ahydrochloric acid rust removing agent, immersing said metal terminal ina neutralizing rust preventing agent.
 24. A method of producing acathode ray tube according to claim 18, wherein said step of removingthe oxide film comprises the step of mechanically shaving off the oxidefilm.
 25. A method of producing a cathode ray tube having a spiralhigh-resistor formed on an inner face of a glass tube which functions asa main focusing lens for an electron gun, wherein the method ofproducing the cathode ray tube comprises the steps of:welding said glasstube to a metal terminal by melting a frit or said glass tube itself,the metal terminal applying a predetermined potential to said spiralhigh-resistor; and flattening the bonded portion of said metal terminaland said glass tube.
 26. A method of producing a cathode ray tube havinga spiral high-resistor formed on an inner face of a glass tube whichfunctions as a main focusing lens for an electron gun, wherein themethod of producing the cathode ray tube comprises the steps of:weldingat least one open end of said glass tube to a metal terminal by meltingsaid glass tube itself, the metal terminal for applying a predeterminedpotential to said spiral high-resistor; and chamfering an inner face inthe vicinity of said open end of said glass tube.
 27. A method ofproducing a cathode ray tube having a spiral high-resistor formed on aninner face of a glass tube which functions as a main focusing lens foran electron gun, wherein the method of producing the cathode ray tubecomprises the steps of:welding at least one open end of said glass tubeto a metal terminal by melting said glass tube itself, the metalterminal for applying a predetermined potential to said spiralhigh-resistor; and making an inner diameter of said glass tube largerthan an inner diameter of said metal part.
 28. A cathode ray tube havinga main focusing lens in which a high-resistor is formed into a spiralshape by a high-resistance material having a thermal expansioncoefficient of 36 to 105×10⁻⁷ /deg. C., on an inner face of a glass tubehaving a tubular structure made of borosilicate glass or soda glassmaterial having a thermal expansion coefficient of 36 to 105×10⁻⁷ /deg.C. and a volume resistance of 1×10¹⁰ to 1×10¹² ohm·cm,furthercomprising:metal terminals which are used for electrical connectiondisposed at both ends of said glass tube; a high-resistor film which isobtained by forming a high-resistor paste into a spiral shape on aninner face of said glass tube and firing at a temperature in the rangeof 420 to 550 deg. C.; and electrodes of an electron gun attached tosaid glass tube whereby the high-resistor serves as the focusing lens ofthe cathode ray tube.
 29. A cathode ray tube according to claim 28characterized in that said cathode ray tube further comprises a focusingvoltage supply portion which is disposed in the vicinity of the centerof said glass tube.
 30. A cathode ray tube having a spiral high-resistorwhich functions as a main focusing lens for an electron gun and which isformed on an inner face of a glass tube, the cathode ray tubecomprising:a metal terminal which is bonded to said glass tube bymelting a frit and which applies a predetermined potential to saidspiral high-resistor, the thermal expansion coefficients of said glasstube, said frit, said metal terminal, and a material of said spiralhigh-resistor being in the range of 36 x to 105×10⁻⁷ /deg. C.
 31. Acathode ray tube having a spiral high-resistor formed on an inner faceof a glass tube which functions as a main focusing lens for an electrongun, wherein the cathode ray tube comprising:a metal terminal which iswelded to said glass tube by melting a frit or said glass tube itself,the welding of said glass tube and said metal terminal being conductedin a reducing gas atmosphere or an inert gas atmosphere, the metalterminal applying a predetermined potential to said spiralhigh-resistor.
 32. A cathode ray tube having a spiral high-resistorformed on an inner face of a glass tube to function as a main focusinglens for an electron gun, the cathode ray tube comprising:a metalterminal having a film for preventing oxidation on said metal terminalwhich is welded to said glass tube by melting a frit or said glass tubeitself, the metal terminal provided to apply a predetermined potentialto said spiral high-resistor.
 33. A cathode ray tube according to claim32 wherein said film for preventing oxidation is formed by one ofdeposition of gold, gold plating, chromium plating, and nickel plating.34. A cathode ray tube having a spiral high-resistor formed on an innerface of a glass tube which functions as a main focusing lens for anelectron gun, the cathode ray tube comprising:a metal terminal which iswelded to said glass tube by melting a frit or said glass tube itselfthe metal terminal applying a predetermined potential to said spiralhigh-resistor, the metal terminal having an oxide film on a surface ofsaid metal terminal, after welding said glass tube to said metalterminal.
 35. A cathode ray tube according to claim 34 wherein theremoval of said oxide film is conducted by reduction in a hydrogen orhydrogen-mixture gas atmosphere.
 36. A cathode ray tube having a spiralhigh-resistor formed on an inner face of a glass tube, to function as amain focusing lens for an electron gun, the cathode ray tubecomprising:a metal terminal which is welded to said glass tube bymelting a frit or said glass tube itself the metal terminal applying apredetermined potential to said spiral high-resistor, the portion ofsaid glass tube which is welded to the metal terminal being flattened.37. A cathode ray tube having a spiral high-resistor formed on an innerface of a glass tube, to function as a main focusing lens for anelectron gun, the cathode ray tube comprising:a metal terminal which iswelded to at least one open end of said glass tube by melting said glasstube itself, the metal terminal applying a predetermined potential tosaid spiral high-resistor, the inner face of the tube in the vicinity ofsaid open end being chamfered.
 38. A cathode ray tube having a spiralhigh-resistor formed on an inner face of a glass tube, to function as amain focusing lens for an electron gun, the cathode ray tube having ametal terminal which is welded to at least one open end of said glasstube by melting said glass tube itself, the metal terminal providingelectrical connection to the spiral high-resistor for applying apredetermined potential to said spiral high-resistor, the inner diameterof said glass tube being larger than an inner diameter of said metalterminal.